to maintain a colony of caulerpa, keep it well pruned so the overall plant mass doesn't get too big. That's why it crashes(and goes to spore), because it limits out.
Halimeda also goes to spore when it crashes, so only keep a token amount of that too.(related to caulerpa believe it or not)
really, anything crashes when it runs out of nutrients. My entire tank of chaeto, which I've had for years finally crashed back to a one-good-handful sized wad down from a whole salt bucket full.
The thing about chaeto is it fizzles out cleanly, and you can pull the dying stuff from the healthy stuff as it goes rather than the whole population tanking at once.